Operational amplifiers usually comprise an input stage and at least one output stage, which is often configured as a class A stage. In this case, the quiescent current in the output stage basically corresponds to the maximum output current. Especially for signals with a high crest factor, that means with a high ratio between a peak value and a root mean square value of the electrical alternating signal, a class A mode of operation results in a high current consumption and therefore in a low efficiency of the amplifier stage or the operational amplifier respectively.
In systems for mobile communication, circuit blocks with operational amplifiers in a receiving path are in a so-called listen mode with a high probability, depending on the used mobile communication system. In this case, the circuit blocks with operational amplifiers are active but do not receive a signal and therefore are not provided with an input signal. Especially for amplifiers with a class A mode of operation this leads to a high current consumption and a low efficiency.
FIG. 9 shows an exemplary embodiment of a conventional class A output stage which, for example, can be used in a Miller operational amplifier. The amplifier stage comprises two signal paths with respective NMOS transistors NB1, NB2, and PMOS transistors PB1, PB2 connected in series. An input INP, INM of the amplifier stage is formed by the control input of the transistors NB1, NB2. The signal paths are connected between a supply voltage tap VDD and a reference voltage tap GND. Taps OUTM and OUTP which are connected between the NMOS transistors NB1, NB2 and the PMOS transistors PB1, PB2, respectively, provide the amplified output signal. A quiescent current through the PMOS transistors PB1, PB2 is controlled by a reference transistor P0 which is connected to the PMOS transistors PB1, PB2 as a current mirror and which is coupled to a reference current input IB for receiving a reference current. The quiescent current corresponds also to the maximum output current of a signal path. The current value depends on a current mirror ratio between the transistors PB1, PB2 and the transistor PB0. By providing an input signal to the input taps INP, INM, for example by a pre-stage or input stage, the current through the signal paths and therefore an output signal at the output OUTM, OUTP is influenced. As mentioned before, the high quiescent current also flows for input signals with small or even no amplitude.
As an alternative, several class AB amplifier stages can be used instead of the shown class A amplifier stage as an output stage. There are different class AB output stages known which comprise a negative feedback or get by without a negative feedback.
Class AB output stages without a negative feedback usually comprise additional parasitic capacitances for the output transistors which reduces the frequency bandwidth of the amplifier stage substantially. Furthermore, an additional current and area consumption of current paths for adjusting a control voltage of the output transistors have an undesired effect in case of a quiescent current mode.
On the other hand, class AB output stages with negative feedback require high bandwidths for the feedback circuit. This again results in a high current consumption.
For applications with very strict requirements regarding a frequency bandwidth, for example wireless local area network, WLAN, e.g. according to 802.11n, it is therefore necessary to increase the current consumption for compensation of bandwidth loss so much that this does not result in a noteworthy advantage regarding the current consumption compared to conventional class A output stages. For this reason, for applications with strict requirements regarding the frequency bandwidth, almost solely class A operational amplifiers are used, that means operational amplifiers with a class A output stage.
It is therefore not known for operational amplifiers to achieve a low current consumption, neither with class A nor with class AB output stages, especially for a quiescent current mode, without reducing a possible output power or a possible frequency bandwidth.